Initially, aromaticity was found in planar, two-dimensional molecules, the most typical and well-known case being benzene. Benzene, a cyclic molecule with six carbon atoms has a ring of resonance that gives the molecule an unexpected increased stability and exceptional chemical behavior. Later, aromaticity was studied in all-metal three-dimencional molecules, the most paradigmatic case being in MAl₄^- clusters (being M a metallic ion such as Li, Na or Cu), in excited states (for instance, the triplet lowest-lying excites state of cyclopentadienyl anion) or in transition states of allowed pericyclic reactions (reactions in which molecules with cyclic geometries are involved in the transition state). However, these aromatic compounds are not stable at all. On the other hand, fullerenes (C₆₀), which have all the indications to be 3D aromatic, to the discomfort of many, they are not, despite being very stable.

We see then that there are examples in which stability and aromaticity do not always go together. Therefore, the definition of aromaticity and how to measure it is still nowadays a critical issue, much debated and that continues with a lot of research behind it. Even close collaborators, formed by experimental and theoretical scientists, have discrepancies about this concept. Discrepancies that have always, however, strengthened the need to better understand and unite the two points of view.

For experimental scientists, like the team of Francesc Teixidor and Clara Viñas, from the Inorganic Materials & Catalysis group, working at the ICMAB, aromaticity should be related to stability, to electrophilic or nucleophilic substitution, and to diatropic ring currents. For theorists (in general, and in particular their collaborators from the Universitat de Girona, Miquel Solà, and from ICREA, Jordi Poater), aromaticity should meet some specific orbital characteristics.

Until now, chemists, in general, viewed that aromacity was related to (1) planar systems with shared delocalized electrons (with conjugated p orbitals) and (2) obeying the famour Hückel rule (which was proposed by Erich Hückel in 1931 to determine if a planar ring molecule would have aromatic properties as a function of the number of electrons) that (3) generated a diatropic ring current when being exposed to a magnetic field. This phenomenon is clearly observed by nuclear magnetic resonance (NMR). However, the ring current is hardly imaginable in 3D systems. In 3D molecules, where exactly should the loop be?

In the paper now published in the Journal of the American Chemical Society (JACS) entitled “Too Persistent to Give Up: Aromaticity in Boron Clusters Survives Radical Structural Changes”, researchers demonstrate experimentally for the first time, focusing on 3D boron blusters, the existence of diatropic currents in a three-dimensional system, and introduce the concept of “discrepancy percentage”. This concept relates the theoretical “nucleus-independent chemical shifts” (NICS) value (a computational method that calculates the absolute magnetic shielding at the center of a ring, which is used to measure the aromaticity of an arbitrary compound) with the corresponding value of the most stable archetype 2D or 3D molecule. The larger the discrepancy value, the less the tendency to stay aromatic for a given molecule.

Results show that 3D closed heteroboranes (e.g. C₂B₁₀H₁₂) have very low discrepancy percentage when compared to 3D closed boranes with the same number of vertices (e.g. [B₁₂H₁₂]^2-). being the latter the aromatic archetype. However, 2D heterocycles show a much larger discrepancy percentage, inducing an easier tendency to lose aromaticity (e.g. diazabenzenes vs. benzene).

"In short, this translates into observing that 3D closed boranes do not give up aromaticity even when adding two extra electrons. This is what the title of the article wants to express "Too persistent to give up". In planar aromatic systems, however, the addition of two extra electrons leads to antiaromaticity" explains Francesc Teixidor, researcher at the LMI group at the ICMAB.

This publication is a relevant contribution to a topic of great importance in the field of Chemistry. Although the scientific community has been dealing wih aromaticity for more than 100 years, the topic is still in vogue and is not fading at all. There are still plenty of molecules to study, both experimentally and theoretically!

Reference Article:

Too Persistent to Give Up: Aromaticity in Boron Clusters Survives Radical Structural Changes
Jordi Poater, Clara Viñas, Ines Bennour, Silvia Escayola, Miquel Solà, Francesc Teixidor
J. Am. Chem. Soc. 2020. Publication Date: April 23, 2020
DOI: 10.1021/jacs.0c02228